1. Field of the Invention
The present invention relates in general to a speed changing gear device in an automatic transmission for motor vehicles such as automobiles, and more particularly to such a speed changing gear device which is equipped with two single-pinion type planetary gear units and one double-pinion type planetary gear unit.
2. Discussion of the Prior Art
An automatic transmission for a motor vehicle is constructed to automatically establish one of a plurality of operating positions which has an optimum speed reduction ratio, depending upon the current speed and load of the motor vehicle. Owing to substantial freedom from manipulation by the vehicle operator, such an automatic transmission is widely used for recently marketed automobiles. Examples of such a vehicle automatic transmission are disclosed in laid-open Publication Nos. 50-32913 and 51-3012 of examined Japanese Patent Applications, wherein three planetary gear units are disposed in series so as to constitute a speed changing gear device. These automatic transmissions are relatively simple in construction, and provide five forward drive positions having respective speed reduction ratios, permitting stepwise speed changes over a relatively wide range of speed reduction ratio (that is, a relatively high ratio of the highest reduction ratio to the lowest reduction ratio). Further, the planetary gear units of the speed changing gear device may have suitable gear ratios (ratio of the number of teeth of the sun gear to that of the ring gear of each gear unit), without having to increase the size of the speed changing gear device. These are some of the advantages of the automatic transmissions having planetary gear units as disclosed in the above-identified publications.
However, the conventional speed changing gear device as used in the known automatic transmission indicated above requires disconnection of one element of the gear device from the input or driven shaft of the transmission, and connection of another element to the input shaft, when the transmission is shifted up from the 2nd-speed position to the 3rd-speed position, from the 1st-speed position to the 2nd-speed position. These disconnection and connection make it difficult to control the speed changing gear device for smooth shifting actions. Described more specifically, there may arise some no-load time interval between the moment of disconnection of one element from the input shaft, and the moment of connection of another element to the input shaft. This no-load time interval results in racing of the vehicle engine, since the input shaft of the transmission is not connected to any of the elements of the speed changing gear device. If the input shaft is connected simultaneously to the two elements of the gear unit for some time, on the other hand, there may arise some shifting shock upon the relevant shifting action of the transmission. Thus, the conventional speed changing gear device requires intricate regulation of the timings at which the transmission input shaft is released from one element of the gear device and newly coupled to another element of the gear device. This problem is conspicuous where a shifting action of the gear device takes place while the engine speed is relatively high, namely, where the shifting action is associated with the 1st-speed, 2nd-speed and 3rd-speed positions which have relatively high speed reduction ratios.